kVA

What is kVA?

kVA (kilovolt-amperes) measures apparent power in an AC circuit. It combines RMS voltage and current. kW (kilowatts) measures active power delivered to loads or the grid. The two are related by the actual power factor under the same operating conditions.

For a sinusoidal single-phase circuit: kVA = volts RMS × amperes RMS ÷ 1,000. For a balanced three-phase circuit: kVA = √3 × line-to-line volts RMS × line amperes RMS ÷ 1,000. Use the correct voltage and current convention before applying a formula to a real connection.

Why apparent power exists

In a DC circuit, power is voltage times current. In an AC circuit, voltage and current alternate, and they can be out of step. The product of their RMS values (kVA) is the total the equipment has to carry. Only the in-phase part does useful work (kW). The remainder, called reactive power (kVAR), moves back and forth between the source and the load. The three are linked by:

kVA² = kW² + kVAR², and power factor = kW ÷ kVA.

Because conductors, windings and power electronics heat according to the current, equipment is rated in kVA: the rating says how much current it can carry at a given voltage, whatever mix of kW and kVAR that current represents.

kVA versus kW: a checked example

A 5 kVA label alone does not tell you battery backup duration, motor-start capability, PV input or grid-export approval. Read those specifications separately.

Worked current examples

• Single phase: 230 V × 21.7 A ÷ 1,000 = about 5.0 kVA.
• Three phase: √3 × 415 V × 27.8 A ÷ 1,000 = about 20.0 kVA.
• Current from kVA, single phase: 5 kVA at 230 V draws 5,000 ÷ 230 = about 21.7 A.
• Current from kVA, three phase: 20 kVA at 415 V draws 20,000 ÷ (√3 × 415) = about 27.8 A per phase.

Check the voltage convention. Single-phase equipment uses the phase-to-neutral voltage, and three-phase formulas use the line-to-line voltage.

A datasheet example: kW and kVA on one model

Qbits datasheets list both a rated output power in watts and a maximum output power in kVA. For the QB-4.6/5/6KTLS single-phase family, the published technical data (checked October 2026) show 4,600 W with 5.06 kVA, 5,000 W with 5.5 kVA, and 6,000 W with 6.6 kVA. The QB 6 to 17KTLC three-phase datasheet shows 6,000 W with 6.6 kVA for the 6KTLC. In each case the kVA figure is 1.1 times the rated watts, which corresponds to a ratio of about 0.91 between kW and kVA at maximum apparent power.

Two cautions follow from the arithmetic, and both are worth confirming with the manufacturer rather than assuming:

• The kVA figure is the apparent-power limit. If the inverter is asked to supply reactive power, the active power it can deliver at the same time is limited by that kVA figure. For example, 6.6 kVA at a power factor of 0.8 allows at most 6.6 × 0.8 = 5.28 kW.
• The kVA rating of one model is not a rule for another. Quote the figures from the datasheet of the exact model.

The datasheets also state a power-factor range for the same models, which the power factor entry explains.

Using kVA in inverter selection

Check the exact inverter datasheet, starting from the on-grid inverter range, for both continuous kW and kVA, permitted power-factor range, phase, output voltage and short-duration overload. A motor can require a higher starting current than its normal-running kW suggests. A site may also have a DISCOM contract-demand rule that uses a different measurement period or unit; inspect the actual tariff and bill.

The solar inverter sizing guide explains the full equipment choice. The single versus three-phase guide explains how the choice between phases changes the arrangement. Avoid a universal “add 25%” home-sizing rule: the safe margin depends on the specified loads, start-up behaviour, product ratings and circuit design. Ask an installer to verify the load schedule rather than choosing solely from a kVA conversion.

Surge and motor starting

Loads with motors or compressors draw a higher current when they start than when they run. How much higher depends on the specific motor and its starter, so the motor’s nameplate or datasheet (locked-rotor current) is the right source, not a generic multiplier. Compare that starting current with the overload or surge capability stated for the inverter, and with how long that capability is available. Continuous kVA does not include surge unless the datasheet says so.

kVA in transformers, generators and UPS

A transformer’s nameplate kVA rating states how much apparent power it can carry at rated voltage within its thermal limits. Power transformers are covered by IS 2026 in India and the IEC 60076 series internationally. A generator and a UPS are rated the same way. When reactive current rises, the current rises with it, so a poor power factor can use up kVA capacity even when kW is modest. The presence or absence of a transformer stage inside an inverter changes its design, as the transformerless versus transformer-based inverter comparison explains.

kVA in utility tariffs

Many commercial and industrial tariffs bill or limit a maximum demand in kVA, and a connection’s sanctioned capacity may be stated in kVA or kW depending on the state and category. Exceeding the contract figure can attract a charge under the tariff order. Read the unit and the measurement interval in your own tariff and bill before sizing, and see the DISCOM entry for who to ask.

Common kVA problems

• A customer assumes a 5 kVA inverter will run 5 kW of load at a lower power factor.
• An off-grid or backup inverter trips when a motor starts because the starting current exceeds its overload capability.
• A site exceeds contract demand because kVA, not kW, is what the utility measures.
• A transformer or inverter runs hot because reactive current is using capacity.
• A quotation converts kVA to kW without stating the power factor assumed.

Best practices

1. Size from the actual load schedule, including starting behaviour, not nominal wattage.
2. Specify continuous and short-duration ratings in the purchase order.
3. State the power factor whenever you convert between kW and kVA.
4. Check the tariff and bill for the unit and interval used for demand.
5. Verify the specification on the exact model’s datasheet.

Where it applies in India

DISCOM tariffs for commercial and industrial consumers commonly use kVA for contract or maximum demand, while a household connection is often stated in kW, depending on the state. Rooftop solar under residential schemes is typically described in kW of installed capacity, with kVA as a secondary inverter specification. Check the tariff order of your state commission for the unit that applies.

Standards and references

Common mistakes

• Treating kVA and kW as interchangeable.
• Dividing or multiplying by an assumed power factor without saying so.
• Applying one model’s kVA figure to another model.
• Ignoring starting current when sizing for motors.
• Mixing phase and line voltages in a three-phase calculation.

Key takeaways

kVA is the apparent power the equipment has to carry. kW is the part that does work. Power factor links them, and the datasheet of the exact model is where the usable numbers live. Size from the load schedule, check the tariff unit, and state your power factor assumption whenever you convert.